1. Field of the Invention
The invention relates to au electro-optic distance measuring device.
2. Description of Related Art
Relevant distance-measuring devices are known from EP 0 205 406, EP 0 313 518, EP-A-1 647 838, WO 97/18486 and EP patent application number 10 405 078, for example. The content of these applications is incorporated in their entirety by reference for elucidating the functioning of the Fizeau method for absolute distance measurement. U.S. Pat. No. 3,424,531 describes a distance measurement device using a light modulator that turns light transmission on and off, similar to a rotating tooth-weel.
FIG. 1 schematically shows a distance measuring device for measuring absolute distance according to the prior art: A light source 101 emits light, typically in the visible or infrared range, with center wavelength λ, the spectral width Δλ of the source being broad enough in order to ensure a low coherence light emission. The parallel light beam emitted by the broadband source 101 illuminates a polarising beam splitter 102, which ensures a linear polarization state for one of the transmitted beams. The polarized beam passes through an electro-optic crystal 103 having electrodes 104 on opposite sides. The incident light beam is polarized at 45° to the main axes of the electro-optic crystal 103, called slow (extraordinary) and fast (ordinary) axis, with different refractive indices ne>nc, respectively. Two waves resulting from the projection on the orthogonal axes of the incident light field propagate in the electro-optic crystal 103 with a 90° polarization angle between them. The electrodes 104 allow to apply an electric field parallel to one of the main crystallographic axis of the electro-optic crystal 103. A sinusoidal electric signal with a frequency f is generated by a signal source 108 and applied to the electrodes 104. This electric field generates a modification of the refractive index difference between the slow and the fast optical axes of the crystal. A phase modulation is thus introduced between the two orthogonal waves. The value of the index of modulation relative to the slow axis αs and to the fast axis αf depends on the electro-optic coefficient r of the for the crystal orientation considered, on the cube of the refractive index of the slow and fast axis respectively, on the distance gap between the electrodes, on the crystal length and on the optical wavelength, and on the voltage amplitude of the electrical signal. The polarization directions(s) along the path of the light indicated by small arrows.
At the output of the electro-optic crystal 103, a quarter wave plate 105 is placed with its axes oriented at 45° with respect to the main axes of the electro-optic crystal 103. The light beam after passing through the quarter wave plate 105 passes on, along the distance to be measured, to reach a target. A corner mirror 106 or other reflecting element is fixed to the target, reflecting the light back to the optical source. After passing a second time through the quarter wave 105 plate, the two orthogonal waves of the returning light are rotated by 90° and cross the electro-optic crystal 103 a second time, now in the opposite direction. The wave, which was modulated the first time along the slow axis, is now modulated along the fast axis, while the wave, which was modulated along the fast axis the first time, is now modulated along the slow axis. The modulation being experienced by the light is the same for the light passing in the forward and backward direction, but is a delayed by the time of flight of the wave on its way to the target and back. The returning light and part of the emitted light are then recombined at the second polarizer output port. The two resulting linear waves can now interfere. The resulting beam, modulated in amplitude according to the interference, is captured by a photoreceiver 107.
Thus, basically, a light beam, from a laser or from a broadband light source, is generated, and guided by a focusing optical unit onto a polarizing beam splitter for linearly polarizing the light, and is subsequently guided onto a measurement path by an electro-optical modulator, a lambda/4 retarder and an exit optical unit. Light returning along the measurement path passes through the elements mentioned as far as the polarizing beam splitter and is guided onto a detector by the latter. An evaluation unit serves for determining the length of the measurement path on the basis of the detector signal.
What is of relevance in the present context is that, in this method, outgoing and returning measurement light is modulated in a modulator. By variation of the frequency of the modulation, a minimum of the intensity of a detected measurement light beam is determined (or substantially synonymously, a zero-crossing of the derivative of the intensity). The length of the measurement path between the measurement device and a retroreflector or a semi-cooperative target is determined from the minimum frequency. A semi-cooperative target returns at least part of incident light along the direction of the incident light, e.g. by diffuse reflection.
Current implementations of Fizeau-principle based distance-measuring devices use electro-optic modulators with bulk crystals exhibiting the Pockels-effect. In order to reach the voltages of several 100 V (over a crystal width of ˜1 mm) required for full modulation, the modulator needs an electrical drive-power of ˜1 W, and the crystal is placed in an electrical resonator. Setting a particular modulation frequency requires mechanical tuning of the resonator, thus limiting the measurement rate (to e.g. 20 Hz).
It is desirable to speed up the measurement by using an integrated optics modulator in a distance measurement device. However, since the measurement principle of the distance measurement device requires the light to pass the modulator twice, in opposing directions, known single pass modulators are not suitable.